1. Field of the Invention
The present invention relates to an apparatus for controlling an electroexpansive actuator which is used in a fuel injection valve of a spark ignition internal combustion engine, a fuel injection apparatus of a diesel engine, or the like.
2. Description of the Related Art
In general, an electroexpansive actuator comprises a plurality of columnar laminated piezoelectric elements having an electroexpansive effect. Each piezoelectric element comprises a PZT.RTM. ceramic obtained by sintering lead zirconium titanate. In manufacturing such an electroexpansive actuator, a direct voltage is applied to each piezoelectric element so as to generate polarization therein, thereby preserving a high electroexpansive effect. However, when a repeated load is applied to each piezoelectric element, the polarization thereof is deteriorated, and accordingly, the electroexpansive effect thereof is deteriorated. For example, as illustrated in FIG. 1, which shows the characteristics of the displacement of a laminate of 80 piezoelectric elements each having a diameter of 15 mm and a thickness of 0.5 mm, when the endurance time period increases, the displacement decreases due to the deterioration of polarization. Therefore, if such an expansive actuator is used in a fuel injection valve of a spark ignition engine or in a fuel injection apparatus of a diesel engine, the amount of injected fuel may decrease.